In one video, thought to have been shot on Tuesday evening, UCP members are seen detaining hundreds of migrants. Many appeared to be families with children, who were then held until border patrol agents arrived.

“This group was so huge we wont [sic] have an accurate count until the [Border Patrol] is finished processing them,” the group wrote. “This needs to stop!!!!! Build the wall.”

Dozens more videos show similar detentions of the migrants – an increasing number of whom are seeking asylum in the US after fleeing gang violence in their home countries.

The organisation says its members “stand up for the rights of every American” and “help keep America safe”.

However, the vigilante detentions on the border have been condemned by the ACLU, which described UCP as a “fascist militia organisation”.

In a letter to New Mexico governor Michelle Lujan Grisham and state attorney general Hector Balderas, the ACLU alleged the detentions were illegal and called for an investigation into the situation.

“[The United Constitutional Patriots’] actions undermine the legitimate efforts of our state’s law enforcement officials to keep New Mexico families safe and they erode community trust,” the letter added.